VEGETABLE CROPS

Virus
Diseases of Snap and Dry Beans

by
T. A. Zitter, Dept. of Plant Pathology, Cornell University and R. Provvidenti
Dept. of Plant Pathology New York State Agricultural Experiment Station, Geneva

Virus diseases can commonly occur on both snap and dry bean crops in New York
State and can result in reduced yields and inferior quality of the product. Because
dry beans have a longer vegetative cycle, viruses. in general, are more destructive
for this crop. Most bean viruses are spread by aphids in a nonpersistent manner,
with insecticides providing little protection against virus spread during the
season. In addition to being aphid transmitted, some bean viruses are seedborne
in bean or in other hosts and can be overwintered in cultivated plants and weeds.
Nematodes can transmit two viruses, which cause only minor plant injury.

Major
Bean Viruses

Bean common mosaic virus (BCMV) is still an
important disease of beans worldwide, but is less of a problem in commercial bean
varieties because many possess genes for resistance. The most common strains are
the type virus known formerly as bean virus 1 (BV-1) and a New York strain (NY-15).
Some, but not all, dry bean varieties are resistant to one or both strains (see
table 1), whereas most snap bean varieties are resistant to both strains.

Typical
symptoms in bean consist of green mosaic and downward cupping along the main vein
of each leaflet (fig. 1). Green veinbanding, blistering,
and malformation are common in leaves of the same plant (fig.
2). Plants are reduced in size, and pods may be mottled and malformed. These
symptoms could be easily confused with those caused by cucumber mosaic virus (CMV).
However, the BCM symptoms persist, whereas those caused by CMV are transitory.
In some dry bean varieties (see table 1) resistance to mosaic is expressed as
a hypersensitive reaction conferred by the I gene, leading to a condition called
"black root." Infection with some strains causes root and stem discoloration,
rapid wilt (systemic necrosis), and eventual death.

BCMV is seedborne in
bean, especially if the seed was produced locally. The virus is rarely present
in wild legumes. The virus is efficiently transmitted by several aphid species
includin the following: bean aphid (Aphis fabae), cowpea aphid (Aphis
craccivora), pea aphid (Acyrthosiphon pisum), potato aphid (Macrosiphum
euphorbiae), and the green peach aphid (Myzas persicae).

BCMV
is most effectively controlled by growing resistant cultivars and using virus-free
seed.

Bean yellow mosaic virus (BYMV), also called bean virus
2 (BV-2), is the most widespread bean virus in New York State and can be found
nearly every year. The symptoms of this disease may vary depending upon the variety,
the virus strain, the stage of growth at the time of infection, and environmental
factors.

BYM symptoms consist of leaf mosaic formed by contrasting yellow
or green mosaic areas (fig. 3, 4). Pods are usually
not affected, but the number of seeds per pod may be reduced.

Table
1. Reaction of selected dry bean varieties to two strains of bean common
mosaic

BYMV
is not seedborne in beans, but may be in other legumes. The most common overwintering
hosts include clovers, wild legumes, and certain flower crops such as gladiolus.
The virus is spread in nature by the same aphid species that are mentioned under
BCMV. Presently, only a few experimental lines and even tewer commercial varieties
are resistant to BYMV, but more will be available in the future.

Cucumber
mosaic virus (CMV) can infect almost 800 plant species, but only a specific
strain can cause a disease of bean and certain other legumes. Symptoms consist
of leaf curl, green mottle and blistering (fig. 5),
and a zipperlike rugosity along the main veins. involving only a few leaves. Symptoms
resemble those for BCMV as noted previously, but plants usually recover and resume
normal growth.

CMV may be seedborne in some bean varieties but, more importantly,
is overwintered in many weed species (chickweed, milkweed, purslane, and clover).
The virus is transmitted by at least 60 aphid species.

Clover yellow
vein virus (CYVV) was previously considered as the severe strain of BYMV
because of many similarities in host range and symptom expression. However, this
virus, in addition to causing a prominent yellow mosaic, malformation, and reduction
in plant size (fig. 6). can induce pod distortion
(fig. 7). In addition to infecting bean, this
virus can be recovered from cucurbits, peas, wild violets, and many cultivated
and wild legumes. As with all the major bean viruses, CYVV is aphid transmitted
and is overwintered in legumes and some ornamentals like garden violet.

Minor
Bean Viruses

Tobacco and tomato ringspot viruses (TRSV and TmRSV)
are often found infecting the crop at the same time. Initial symptoms are very
prominent and consist of chlorotic ringspots, mottling, malformation, and necrosis.
Plants recover from this severe stage, and later foliage may appear completely
normal. These viruses are not seedborne in beans, but may be in other legumes.
Unlike all other bean viruses, these viruses are spread by the nematode Xiphinema
americanum, and the viruses can overwinter on many weed species in the state
without expressing virus symptoms. Resistance is available in some bean varieties.

Alfalfa mosaic virus (AMV) can cause a disease of beans noted
by scattered bright yellow dots on the leaves, which are characteristic for this
virus when it infects other plants. AMV does not appear to reduce yield or quality
of the product. The virus may be spread by many aphid species.

Soybean
mosaic virus (SMV) induces symptoms closely resembling those caused by BCMV.
Most leading commercial snap bean varieties are resistant to SMV, with only a
restricted number showing susceptibility. SMV is spread by several aphid species,
and the major source of infection is soybean, for this virus is commonly seedborne
in that crop

Watermelon mosaic virus 2 (WMV-2) causes symptoms
similar to those incited by some strains of BYMV. Most commercial snap bean varieties
are resistant to WMV-2. The leading source of inoculum is some clover species.
The virus is not known to be seedborne and is spread in nature by aphids.

Cooperative
Extension, New York State College of Agriculture and Life Sciences, New York State
College of Human Ecology, and New York State College of Veterinary Medicine at
Cornell University, and U.S. Department of Agriculture, cooperating. In furtherance
of acts of Congress May 8 and June 30, 1914, and providing equal opportunities
in employment and programs. Lucinda A. Noble, Director. 11/84 CRP 4 M 9134d